Wolfgang Amadeus Mozart - 21 in 1777

By the time Mozart turned 21, he'd written more than many composers could hope to write in a lifetime. His first surviving composition is an Andante in C for keyboard, composed when he was just 5 years old in 1761. He tackled his first symphony, No. 1 in E flat, when he was 8, wrote his first opera, Apollo et Hyacinthus, and his first piano concerto when he was 11. The incredible Exsultate, jubilate flowed from his pen when he was 16, and he'd already completed his Symphony No. 28 by the time he was 18, with around 40 other symphonies and symphonic fragments under his belt.